There are a large variety of methods for producing metal and ceramic foams or similar porous metal structures starting from liquid or powdered metals [1]. Currently there are two ways for directly foaming metals. The first of them involves melting the Al matrix metal, adding reinforcing particles to the melt (5-20% SiC or Al.sub.2 O.sub.3) and injecting gas (air, nitrogen, argon) into the melt using a rotating impeller. The second technique for directly foaming melts is to add a foaming agent to the melt. The foaming agent decomposes under the influence of heat and releases gas, which then propels the foaming process [1-3]. Another method, which was developed some years ago in the Ukraine, exploits the fact that some liquid metals form a eutectic system with hydrogen gas. As the melt cools bubbles of hydrogen are released [4, 5].
Metal and ceramic foams can also be fabricated using open porosity polymer foams as a starting point. The polymer foam is filled with a slurry of heat resistant material, e.g. a mixture of mullite, phenolic resin and calcium carbonate. After drying the polymer is removed and molten metal is cast into resulting open voids. After removal of the mold material (e.g. by water under high pressure) metallic foam is obtained, which is an exact image of the original polymer foam [1]. Polymer foams can also be used in a deposition technique. Metal is deposited on the polymer foam, then the polymer is removed by heating.
Another method for foam calls for casting around inorganic granules of hollow spheres of low density or by infiltrating such materials with a liquid melt [6]. Powder metallurgy methods [1, 7-8] include mixing powders with a foaming agent, compaction of the powder blend into a dense precursor material and foaming of the precursor material by heating it to its melting temperature. Foams can also be produced by preparing a slurry of metal or ceramic powder mixed with a foaming agent. The slurry becomes more viscous and starts to foam during drying in a mould at elevated temperature [1, 9-10].
Most foaming techniques work well for lightweight low-temperature metals, predominantly aluminum and its alloys, but can not be used for fabrication of high-temperature metallic or ceramic foam. However, there is a need for a universal method, which could be applied to the fabrication of foams from any material--metals, ceramics, intermetallics, composites. The vast majority of existing techniques do not allow rigid control of cell shape and size. Thus there arises a wide variation of cell sizes, an uneven distribution of cells in the foam volume and, as a result, a wide scatter in mechanical characteristics.